1. Field of the Invention
The present invention relates in general to optical disk storage systems and more particularly, to a method and apparatus for providing focus control on a multi layer disc.
2. Description of the Related Art
In recent years, optical disk devices have been used to record or reproduce large amounts of data. Optical disks are storage mediums from which data is read and to which data is written by laser. Each optical disk can store a large amount of data, typically in the order of 6 gigabytes. Such optical disk devices are under active technical developments for achieving higher recording density.
Current rewritable optical disks include spiral-shaped groove tracks having concave and convex portions, typically referred to as pits and lands respectively, formed on the surface of a disk substrate. On the surface of the substrate, a thin film that includes a recording material as a component is attached. During fabrication of the disks, concave and convex portions are often formed on the recording surface, simultaneously with the formation of guide grooves for tracking control, so as to record address information of each sector.
Each track of the optical disk is irradiated with a light beam having a predetermined recording power, so as to form recording marks on the recording thin film. The portions irradiated with the light beam (i.e., the recording marks) have different optical characteristics (reflection characteristics) from the other portions of the recording thin film. Thus, the recorded information can be reproduced or read by irradiating the track with a predetermined reproduction power and detecting light reflected from the recording film.
Currently, dual layer optical discs are available. On the surface of the substrate, a first semi-reflective layer that includes a recording material as a component is attached. A second, reflective layer that also includes a recording material is attached to the first layer. Because the first layer is semi-reflective, the light beam irradiated onto the disk may be focused onto either the first or second layer, so as to retrieve data located on each of the layers. For example, movies are increasingly being recorded on such dual layer discs. During playback, information may be retrieved from data located on both layers of the disc. As a result, the disc controller has to switch back and forth between reading of data from both layers. To ensure minimum interruption during playback, such a dual layer jump has to be executed in the shortest possible time. Otherwise, undesired results such as delays and frozen screen effects may occur.
To read data from each of the layers, one of two approaches is typically implemented. The first approach involves moving the lens from either of the layers to a known or reset position. A normal focus search is then initiated. When the target layer is found, typically through monitoring of the focus error signal zero crossing, the focus servo is turned on. Data is then read from the target layer using the appropriate focus control. This approach involves physically moving the focusing lens to a reset position, which subjects the servo control circuitry to external disturbances. In addition, because the focusing lens has to be reset each time reading from another layer is required, the time for executing the jump is much longer than desired.
The second approach involves moving the focusing lens up or down. When lens motion is detected, the lens focusing circuitry is enabled. Like the first approach, this method requires the performance of an arbitrary and disconnected layer jump, which typically involves a delay of 100 ms or more. As a result, system performance is unnecessarily compromised.
Accordingly, there is a need in the technology to overcome the aforementioned problems.